It is desirable to use a snubber circuit across the switching elements (bipolar, FET or IGBT transistors) of switchmode power supply converters to reduce the energy stress on the switching device, to reduce power loss and to minimize the rate of change of turn off voltage to thereby minimize EMI generation and inductive overshoot.
A conventional approach for minimizing switching losses is to place a resistor in series with a capacitor across the switching element. Such an arrangement provides both turn off snubbing and damping of voltage oscillations across the switching element but it incurs a relatively high power loss.
An alternative approach is to place a diode in series with a capacitor across the switching element as for example in Schnetzka, II et al. U.S. Pat. No. 5,075,838 and Pruitt U.S. Pat. No. 4,691,270. Recovery of the energy stored in the capacitor may be achieved by providing a discharge path through a transformer as best illustrated in Pruitt. However, the approaches used in Pruitt and Schnetzka cannot practically be used with single switch forward, flyback or duty converters at high frequency of operating currents and high duty cycles.
It is therefore an object of this invention to provide an improved snubber circuit which features low loss recovery of the energy absorbed by the snubber suitable for single or two switch forward, flyback, duty or other isolated converters.
It is also an object of the invention to provide such an improved snubber circuit wherein the snubber capacitor and the series diode can be placed directly across the switching element as is desired when using very high speed switching elements.